The present invention generally relates to guide drum assemblies, and more particularly to a guide drum assembly for use in a rotary head type magnetic recording and/or reproducing apparatus.
For example, a guide drum assembly of a video tape recorder comprises a guide drum made up of a rotary drum and a stationary drum. The rotary drum is rigidly fixed to a rotary shaft which penetrates a center hole in the stationary drum and is driven by a motor to rotate the rotary drum. A pair of rotary magnetic heads, for example, are mounted on the rotary drum and rotates together with the rotary drum. A magnetic tape is wrapped obliquely around an outer peripheral surface of the guide drum for a predetermined angular range and is transported in a tape transport direction. Signals are recorded and/or reproduced on and/or from the magnetic tape by the rotary magnetic heads.
Basically, there are two methods of driving the rotary shaft to rotate the rotary drum. According to a first method which is often referred to as the belt drive system, a pulley is fixed to an end of the rotary shaft, and a belt is provided between the pulley and a driving shaft of the motor which is provided independently of the guide drum assembly. Hence, the rotary drum is rotated indirectly by way of the belt. According to a second method which is often referred to as the direct drive system, a rotor of the motor is mounted directly on the rotary shaft, and the motor is an integral part of the guide drum assembly. Thus, the rotary drum is rotated directly by the rotor.
The belt drive system has a low frequency transfer characteristic, and for this reason, the cogging and torque ripple of the motor virtually do not affect the rotary drum. However, because of the low frequency transfer characteristic, it is impossible to servo control finely the rotation of the rotary drum, and it is difficult to eliminate a jitter in the low frequency range. For these reasons, the direct drive system is popularly used in the video tape recorder and the like for rotating the rotary drum.
FIG. 1 shows a conventional guide drum assembly of the video tape recorder. A magnetic tape 1 is wrapped obliquely around an outer peripheral surface of a guide drum 4 for a predetermined angular range defined by a pair of guide poles 2 and 3. The magnetic tape 1 is transported in a direction A, and rotary magnetic heads H.sub.1 and H.sub.2 mounted on a rotary drum of the guide drum 4 rotate at a high speed in a direction B together with the rotary drum. A rotary shaft 6 is rigidly fixed on the rotary drum of the guide drum 4, and the rotary drum is rotated by driving the rotary shaft 6. Generally, the rotary shaft 6 is made of stainless steel or the like. The heads H.sub.1 and H.sub.2 alternately scan the magnetic tape 1 obliquely to a longitudinal direction of the magnetic tape 1 to record and/or reproduce signals on and/or from tracks of the magnetic tape 1.
Because the rotary drum of the guide drum 4 rotates at the high speed, an air flow is generated along the periphery of the guide drum 4 as indicated by arrows in FIG. 1, and a so-called air film 5 is formed between the outer peripheral surface of the guide drum 4 and the magnetic tape 1. The thickness of the air film 5 is not uniform throughout the entire predetermined angular range. As indicated by a curve I in FIG. 2, the thickness of the air film 5 is largest at an entrance portion C where the air flow enters with respect to the guide drum 4 along the tape transport direction A, and gradually decreases toward an intermediate portion D and an exit portion E. There is a sudden decrease in the thickness of the air film 5 at the exit portion E due to a vacuum caused by the air flow in a direction F. In other words, even though the magnetic tape 1 is floating with respect to the guide drum 4, the magnetic tape 1 abruptly approaches the outer peripheral surface of the guide drum 4 at the exit portion E. The heads H.sub.1 and H.sub.2 are mounted on the rotary drum of the guide drum 4 at such positions that the heads H.sub.1 and H.sub.2 project from the outer peripheral surface of the guide drum by a distance greater than a maximum thickness of the air film 5. As a result, the heads H.sub.1 and H.sub.2 hit the magnetic tape 1 harder at the exit portion E.
When the heads H.sub.1 and H.sub.2 hit the magnetic tape 1 at the exit portion E, a resistance is generated with respect to the rotation of the heads H.sub.1 and H.sub.2, that is, the rotation of the rotary drum of the guide drum 4. Hence, the guide drum 4 is subject to a periodic external disturbance of 60 Hz in a pulse form, for example, and the rotary shaft 6 undergoes a torsional vibration.
In the conventional guide drum assembly, the rotary shaft 6 undergoes a large torsional vibration when a harmonic frequency of the external disturbance coincides with a natural resonant frequency f.sub.0 of the rotary system. In this case, the rotational speed of the heads H.sub.1 and H.sub.2 deviates, and a jitter is introduced in a reproduced signal obtained from the heads H.sub.1 and H.sub.2. Therefore, a conspicuous jitter is generated in a reproduced picture, at a frequency in a vicinity of the natural resonant frequency f.sub.0, and there is a problem in that it is difficult to obtain a reproduced picture of a high quality.
A Japanese Laid-Open Utility Model application No. 1-61610 discloses a guide drum assembly employing the direct drive system for eliminating some of the problems of the conventional guide drum assembly described heretofore. According to this proposed guide drum assembly, an inertial body (weight) having a predetermined mass is attached through a resilient member to the rotor of the motor which drives the rotary drum. The resilient member has both viscosity and elasticity. Hence, a combination of the inertial body and the resilient member acts as a vibration absorbing system (damper) for absorbing to a certain extent the torsional vibration of the rotary shaft which rotates the rotary drum.
However, in the proposed guide drum assembly, the rotor is rigidly fixed to the rotary shaft. For this reason, the cogging and torque ripple of the motor generated by its rotor are transmitted to the rotary drum by way of the rotary shaft, and as a result, the jitter is generated as the unstable rotation and the vibration of the rotary drum. In addition, even by use of the resilient member, it is impossible to completely eliminate the torsional vibration of the rotary shaft. Hence, due to the remaining torsional vibration which cannot be eliminated, there are problems in that the rotary drum undergoes unstable rotation and a stator of the motor vibrates because of a counter torque of the rotor.